In recent years, for safer driving, technologies for detecting a mobile object approaching a vehicle, in particular, other vehicle or the like approaching from behind the vehicle in a traveling direction, are suggested for the purpose of notifying a driver that the other vehicle is approaching.
For example, according to a technology of Patent Document 1, a vehicle running diagonally behind is detected based on whether or not an optical flow derived from a taken image conforms to an optical flow corresponding to a traveling speed of an own vehicle sensed by a speed sensor.
Patent Document 2 discloses a technology for recognizing other vehicle even when driving in dim conditions. According to the technology of Patent Document 2, in terms of images of behind and/or side of an own vehicle, a movement of a point in a diverging direction thereof from a focus of expansion is detected as an optical flow between two images preceding and succeeding each other by a predetermined period of time. Then, a position of the focus of expansion of the optical flow is changed in accordance with an amount of movement calculated based on a traveling direction information and a traveling distance information of the own vehicle.
Patent Document 3 discloses a technology for recognizing a mobile object in surroundings of an own vehicle based on an obtained optical flow even when the vehicle is not traveling in a straight manner. According to the technology of Patent Document 3, the optical flow obtained from a time-series image is classified in multiple groups of the optical flow having continuous changes. The optical flow is corrected based on a moving direction of the own vehicle assumed from a speed sensor and a yaw rate sensor. Then, the mobile object is extracted from the group of the corrected optical flow and a relative speed is calculated.    Patent Document 1: JPH6-130076(A)    Patent Document 2: JPH9-86314(A)    Patent Document 3: JPH6-282655(A)